NOTE: This spoiler was sent in by Chapanchuriwho says..."I happen to love this movie, its intriguing plot twists and turns. It required a pretty high level of attention, but it completely pays off."

Alex Beck, a doctor who is very much in love with his wife, Margot, since he's known since childhood takes a trip with her to Lake Charmaine which they go to with some frequency. One night, after taking a swim and arguing briefly about Alex's sister Anne, an avid horserider, Margot wanders off into the forest nearby where Alex hears her scream. Following after her, he is knocked out cold by an assailant, falling into the lake.

Eight years later, near the anniversary of his wife's murder, Alex is still a practicing doctor and has been cleared of his wife's murder, though he is still regarded suspiciously by the police--however his wife's murder has been attributed to that of a known serial killer. While doing some investigative work on Alex's land near Lake Charmaine, the police have recently unearthed two bodies and have reason to suspect that Alex may be involved, so they approach him with questions--like, how did he manage to get out of the lake if he was knocked unconscious?--and also take his blood for DNA evidence. This makes Alex, who is still in mourning over his wife, begin to obsess over what happened eight years ago. He speaks to his sister Anne's lesbian lover Helene (played magnificently by Kristin Scott Thomas) who is sympathetic but also tells him that he should try to move on. Instead, like he does every year on the anniversary of his wife's death, he plans to visit his wife's parents in a kind of macabre ritual. In addition to the two bodies showing up on his land, Alex now has to contend with another mystery when an anonymous e-mail pops up telling him to log onto a particular website the next day, and not to tell anyone about it.

Alex is a pretty well-respected doctor--when one of his patients, a thug named Bruno, brings in his son, Alex is the only person who can calm him down. Bruno, who has brought in his hemophiliac son, only trusts Alex with his child. Later, when Alex logs into the website the anonymous e-mail directed him to, which is not available until the exact time stated in the e-mail, he sees a closed circuit live feed from a shopping mall focused on two escalators. He notices a woman walking slowly to the middle of the screen, who turns around and looks into the camera right at him. The woman looks exactly like Margot, which confuses Alex and impassions him to find out what's going on.

On the anniversary of his wife's death, Alex visits and asks his wife's father, who is a policeman and identified his daughter's body at the scene of the crime, about details of the crime. He asks about her injuries, particularly around her face because he believes her father may have been mistaken in identifying his daughter's body. Margot's father is angered and tells Alex to drop it and move on with his life. Alex remains obsessed however and meets up with Helene to discuss the anonymous e-mail. At this time, we also see that Alex's sister, Anne, is indeed an avid horserider and a big part of a foundation headed by Gilbert Neuville, a rich horseowner, who created it in honor of his son who himself died many years ago.

In the meantime, we see that there are some seedy elements who are tracking Alex and have actually managed to hack into his computer unbeknownst to him. They believe Margot is alive. The police, who are still investigating the two bodies found recently come back to speak to Alex about some pictures they'd found recently. These pictures are of Margot, beaten and bruised, and were found in a safe deposit box owned by Margot, the key of which was found in one of the dead men's pockets. The police accuse Alex of beating Margot, and Alex is incensed. These bruises, that she acquired shortly before she died, were explained to Alex by Margot as resulting from a car accident she had while Alex was away for months on a business trip. The police do not seem to believe him when he explains this to them. Alex himself is even more confused by this and another e-mail he receives stating that he should log into another website at a certain time with a User ID of "concert" and a password of "year".

Investigating the matter of Margot's bruises and the pictures, Alex approaches another friend of theirs, Charlotte, a photographer. She is very contrite and admits that Margot asked her to take the pictures, but that Charlotte never believed that Alex abused her. Alex goes back to his own apartment to log into the website the second e-mail has directed him to, with his friend Helene by his side, but he can't get to it. At the same, he notices that there is odd activity on his computer and deduces correctly that his computer has been hacked and that he is being watched. Frustrated and at a dead end, he gets burst of inspiration, and in a particularly beautiful moment in the movie, runs to an internet cafe with his dog to the song "With or Without You" by U2. He realizes that the User ID and password in the e-mail were hints to him. Told to leave his dog outside the internet cafe, he quickly logs in with the User ID "U2" and year "1995" where he is then told to show up in a particular park at 5pm the next day.

At the same time, the seedy elements tracking Alex have shown up to Charlotte's (the photographer) loft and torture her for information, eventually killing her. Discovering the murder the next day while Alex is at work in his clinic, the police find that the murder weapon used on Charlotte is in Alex's apartment. Alex's lawyer, Elysabeth Feldman, calls Alex to tell him that the police are after him and to sit tight and not say anything. Instead, Alex flees his building with the police in hot pursuit. After crossing traffic and crashing a street fair, Alex hides out in a dumpster and calls the one person who he knows can help him: Bruno, the thug who he helped at the hospital with his son.

Bruno arrives in a big black Escalade with a friend of his, and the three of them go to a private investigator named Bernard Valenti because his was one of the last names mentioned in Margot's datebook. Valenti tells him that Margot was apparently having an affair with a young man who was accused of killing Gilbert Neuville's son, Phillippe and that Margot had given him an alibi. Alex and Bruno and Bruno's associate find the young man who reiterates the story, stating that Margot and he were lovers. Alex bluffs and says he knows that to not be true, and the young man backs down and just says that Margot was one of the only people ever there for him. Alex also decides to get Margot's autopsy report and photos to see for himself what Margot's body looked like at death, to see if it was her--it turns out the photos are missing.

Alex, who is trying to make the 5pm deadline at the park, finally manages to make it there where he waits on a bench. The seedy people responsible for killing Charlotte are watching him and watching for Margot to show up. The seedy people mistake a woman with stroller for Margot, and then the audience sees that Margot is in fact at the park and watching Alex. But when Margot sees that the seedy people are out there, she immediately leaves the park. Alex catches a glimpse of her and attempts to catch up with her, but as he does, the seedy people kidnap him in a van, hold him down, and begin to torture him for information. Suddenly, the van stops as Bruno and his friend hijack it and kill Alex's kidnappers and flee the scene with him.

At the same time, we see that one of the policemen, a detective named Detective Levkowitch, is one of the few people who believe Alex's innocence. For one, he questions who has been paying for Margot's safe deposit box the last eight years. And why, if he had killed Charlotte, had he gone to work the next day like everything was normal? And why would he request the autopsy report? Detective Levkowitch himself gets the autopsy report and notices some things that seem amiss. He approaches Helene to tell her that he doesn't believe Alex is guilty of anything. Helene confronts Anne, her girlfriend and Alex's sister, and Alex's sister admits that she has stolen the photos of Margot having been beaten. Anne says that she did it because she was afraid for Margot because she had been having an affair with Phillippe Neuville, the young man who died several years earlier. Helene says that Anne did it because she knew that if she didn't protect Phillippe's reputation, Anne would never be able to ride the horses at Gilbert Neuville's stables ever again. Anne and Alex end up on the phone together, and Alex is coaxed out of hiding (he had originally been thought to be boarding a plane to Buenos Aires because Margot bought and sent him a ticket along with some flowers--we see Margot attempting to board, but when she sees that the police are after him because of Charlotte's murder, she doesn't board the plane either).

The detectives, who now believe he is innocent, show Alex the autopsy report which states that the body they found was inches taller than Margot was in real life, and was also that of a habitual drug user--meaning that it couldn't have been Margot's body. Alex then realizes he needs to confront Margot's father, who had originally identified the body.

When he goes to Margot's father's house, the father pulls a gun on him, feels him up to see if he is armed or carrying anything, and then tells him to sit down. He finally tells his story. Phillippe Neuville was actually a child rapist--the boy that Margot had exonerated in Phillippe's eventual death, had actually been raped by Phillippe and approached Margot for protection. Margot, who had worked for the Neuvilles, appealed to Phillippe to confess by asking him to come over to her house one night. When she asked him to confess, he responded by beating her. Margot's father, who was waiting in the house to arrest him once Phillippe made his confession instead shot and killed him. The audience now sees that Alex was actually wired and we see the policemen's reaction to the father's confession. At this point though, the wire gets static and they can't hear anything. We then hear that Margot's father, who believed Phillippe's father, Gilbert Neuville capable of turning the whole police department against them, state that he sent Margot to Spain for her own safety after staging her death (by using the unclaimed body of a junkie and then cutting up her face so that no one could recognize her). He also told Margot that Alex had died so that Margot would stay away from him.

Then Margot's father admits that Alex's father, who had died years previous to Margot's death (well, now we know it is a disappearance) had also discovered the Phillippe had been raping another child who worked at the stables (Alex's father apparently was another stableworker). After lodging a complaint with Margot's father, weeks later, Margot's father, Alex's father, Gilbert Neuville, and another associate went out hunting where Gilbert Neuville had Alex's father murdered. At this point, it looks like Alex might kill Margot's father but he decides against it and instead goes out to meet the police, who he knew had heard everything. Closing the door of the house behind him, we hear Margot's father killing himself.

The case now closed, and Gilbert Neuville now under arrest, Alex goes back to Lake Charmaine to think about what happened and to mourn once again. As he drives up there, we hear the piece of the conversation that went missing when the wire got static. Margot's father had deliberately turned up the static on the TV to drown out the wire, which he realized Alex was wearing when he felt him up. He then states "Alex, I know you're wearing a wire and I will tell you what really happened." It turns out that after Phillippe beat her up, Margot actually shot Phillippe and her father drove up to the scene afterwards and then concocted the plan to stage her death. He then killed the two people who helped stage the death (the two bodies found at the beginning of the movie) and the one mistake he made was he forgot to take the safe deposit key from the pocket of one of the dead men. Margot's father decided to take the fall for Phillippe's murder.

At Lake Charmaine, we see Alex approach an old tree that he and Margot used to frequent in order to carve the number of years of their relationship into the bark. As he stands there, Margot walks up behind him. He finally hears her footsteps, drops to his knees where Margot catches him, stands him back up, and embraces him. The movie closes with an image of the two of them as children, holding hands and jumping into the water together.